1. Field of the Invention
This invention relates to a technique for obtaining a multifilament composite material, typically for use as a strength member.
2. Description of the Prior Art
Strength members are used in a wide variety of applications, including cables for telecommunications and other uses. For example, a lightguide cable sheath provides mechanical protection and tensile stiffness to optical fibers. High tensile stiffness is desirable for obtaining a high pulling-load rating without exceeding the maximum allowable fiber strain. One successful cable design is shown in U.S. Pat. No. 4,241,979, coassigned with the present invention. In that design, two layers of stainless steel wires, each embedded in high density polyethylene (HDPE), are helically applied as strength members in the crossply sheath design. An unreinforced HDPE sheath would shrink about 6 percent along its longitudinal axis during the sheathing process. If this shrinkage is not controlled, the fibers would suffer high microbending losses. The steel wires provide the compressive stiffness needed to control this shrinkage.
However, in many applications, it is desirable to have an all-dielectric strength member. A strength member qualified for this application should have both high tensile and compressive stiffness. The high tensile strength requirement is met by unimpregnated fiberglass roving (bundle of fibers with little or no twist) or aromatic polyamide (e.g., KEVLAR, a trademark of E. I. Dupont) yarn. However, their compressive stiffness is low because of the yarn-like structure. The filaments comprising the yarn or roving are unable to support the compressive loads that develop upon cooling of the HDPE sheath and thus buckle. This results in an initial low tensile stiffness region or "knee" in the force-strain response of the cable which is contrary to the design intent of a reinforced sheath.
A continuous filament fiberglass roving has not only high tensile stiffness but is also nonconductive, inexpensive, has a thermal coefficient of expansion compatible with optical fibers, and shows no loss in mechanical properties at the cabling temperature. Therefore, it is an excellent candidate as a strength member. To obtain the required compressive stiffness, however, the roving must be impregnated to promote coupling among the filaments. KEVLAR yarn is also nonconductive, but likewise requires improved compressive stiffness for lightguide cable and other uses.
Commercially, impregnation of E-glass roving has been achieved by a process called pultrusion. In this process, the roving is passed through an epoxy resin bath followed by a long heated die for curing. Because of the long die, this process is relatively slow (processing speeds typically of about 1-2 meters/minute), and hence relatively expensive.
Attempts have been made by the present inventors to extrude nylon over E-glass roving. It was found that the nylon did not penetrate the filaments, and therefore this process was not pursued. Other workers have attempted to coat multistrand carbon fibers with an ultraviolet curable acrylate resin. However, inadequate penetration also resulted. The problem of coating multifilament bundles is in contrast to the coating of single optical fibers, wherein coating application and radiation curing are known in the art. For example, optical fibers are coated with epoxy acrylates, silicones, etc., and cured with ultraviolet light.